Surface preparation of an implant

ABSTRACT

A process of forming a prosthetic implant component, such as the femoral component of a knee replacement prosthesis. The process comprises the steps of: (i) forming a prosthetic component having a shape at least approximating the desired final shape of the component from a metal alloy; (ii) subjecting the component to a relatively elevated temperature and pressure followed by a cooling regime; (iii) machining the surface of the component; (iv) polishing the surface of the component.

FIELD OF THE INVENTION

The present invention relates to a process of forming a surgicalimplant, and implants, particularly joint replacement implants, formedthereby.

BACKGROUND OF THE INVENTION

It is well known to use prosthetic joint replacements in patients withvarious kinds of disorders affecting the joints, including degenerativedisorders, such as severe osteoarthritis.

Over the years; a vast array of materials have been developed andutilised in the construction and manufacture of such prostheses. This ispartly because the knowledge base regarding materials, and relevantlybiocompatible materials, has been growing. It is also because, despitetechnological advances, there are a continuing number of complicationsassociated with joint replacement prostheses with which surgeons andpatients must grapple. As a result, surgeons and other inventors in thefield have had, and are still challenged with, an ongoing quest toimprove on the ease of insertion of the prostheses, to reduce theincidence of long and short term complications associated with usingthem, and to improve on the longevity of both the bio-prostheticinterface and the prostheses themselves.

Certain metal alloys, such as CrCoMo alloy, are widely used for thebearing surfaces of prosthetic joints in humans. Attractive features ofsuch alloys include their biocompatibility, their relatively high wearresistance, and their relative ease of manufacture through casting andmachining. Wrought CrCoMo alloy bar is also used, especially for femoralheads.

Following casting, the CrCoMo alloy is often Hot Isostatic Pressed (HIP)to reduce grain size and reduce the internal porosities. Theseporosities are exposed during machining and create holes on the surface.During cooling and the formation of the grains, carbides such as siliconcarbide and molybdenum carbide, can, however, precipitate at the grainboundaries and within the grains. These carbides are extremely hardcompared to the surrounding CrCoMo alloy and after machining andpolishing of the articulating surface usually stand above the surface.While the carbides only stand above the surface by a relatively smalldistance, their presence serve to increase the wear of complementarysurfaces adapted to engage the CrCoMo surface.

In addition, the process of machining and polishing the CrCoMo alloycomponents exposes the parts to a variety of abrasive and potentiallytoxic compounds and particles. While every attempt is made to removethese foreign particles, an implant made of inherently porous materialis difficult to clean perfectly. Studies of the surface of commerciallyavailable implants have shown there to be residual scratches, aluminiumoxide abrasive particles and other foreign matter in an apparentlyclean, polished component. FIG. 1 is a scanning electron micrograph(SEM) of such a surface revealing the scratches, pores and other foreignmatter on a surface that had been polished and cleaned using knownstandard techniques.

Polishing is also a problem as it can alter the dimensional accuracy ofthe component. While the life of a bearing surface of a component may bemodelled by finite element analysis (FEA), it is generally assumed thatthe component is “as designed”. Studies of commercially available partsthat have been hand polished show, however, great variation in the radiiof curves on the bearing surfaces. Variations greater than 20% have beennoted in some cases. Such variation can lead to the implant notperforming in the manner expected of it from FEA.

Coated implants, such as ceramic coatings and PVD Nitride coatings, havealso been tried in a bid to reduce the wear of the material surface,such as the polyethylene component, that is adapted to articulate withthe CrCoMo alloy surface in an implant. These coatings have generallyfailed due to inadequate adhesion in the highly aggressive environmentof joint replacement. The presence of carbides in the implant may alsoadversely affect the process of coating wherein the coating does notadhere to the carbides or other contaminants on the implant. This leadsto areas of weakness in the coating and therefore adversely affects thebond strength of the coating. In addition, coating a roughened surfaceoften only serves to harden the roughened details.

The present invention is directed to a new method of preparing aprosthetic that preferably does not suffer the problems of the priorart.

Any discussion of documents, acts, materials, devices, articles or thelike which has been included in the present specification is solely forthe purpose of providing a context for the present invention. It is notto be taken as an admission that any or all of these matters form partof the prior art base or were common general knowledge in the fieldrelevant to the present invention as it existed in Australia before thepriority date of each claim of this application.

SUMMARY OF THE INVENTION

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

According to a first aspect, the present invention is a process offorming a prosthetic implant component, the process comprising the stepsof:

(i) forming a prosthetic component having a shape at least approximatingthe desired final shape of the component from a metal alloy;

(ii) subjecting the component to a relatively elevated temperature andpressure to relatively reduce the grain and pore size of the metal;

(iii) machining the surface of the component;

(iv) again subjecting the machined component to a relatively elevatedtemperature and pressure; and

(v) polishing the surface of the component.

According to a second aspect, the present invention is a process offorming a rosthetic implant component, the process comprising the stepsof:

(i) forming a prosthetic component having a shape at least approximatingthe desired final shape of the component from a metal alloy;

(ii) subjecting the component to a relatively elevated temperature andpressure followed by a cooling regime;

(iii) machining the surface of the component;

(iv) polishing the surface of the component.

In one embodiment, it will be appreciated that some of the process stepsmay be performed as part of a batch process. For example, in steps (ii)and (iv) a plurality of components may undergo these steps togetherrather than sequentially.

In one embodiment, the metal alloy is a chromium alloy. In a furtherembodiment, the metal alloy is a CrCoMo alloy. The composition of themetal alloy preferably meets the standard specifications of ASTM F 75and ISO 5832/4. These Standards specify the following composition:Carbon C 0.35% max. Chromium Cr 27.00-30.00% Silicon Si  1.0% max. IronFe 0.75% max. Nickel Ni 0.75% max. Manganese Mn 1.00% max. Molybdenum Mo5.00-7.00% max.    Cobalt Co remainder

In step (i), the component is preferably formed by casting the metalalloy in the shape at least approximating the desired final shape. Inone embodiment, the component can be cast such that a layer of about 400microns needs to be machined from the component. In other embodiments,it can be envisaged that the component is cast so as to require more orless material to be removed during the machining step.

In step (ii), the component preferably undergoes a hot isostaticpressing. The hot isostatic pressing (HIP) is preferably carried out atabout 1280 degrees Celsius in an inert gas at about 100 atmospheres ofpressure. The cooling regime of the second aspect may comprise solutionannealing, wherein the component is cooled rapidly, in 100 degree stepsto a temperature below 800 degrees Celsius. This reduces the grain andpore size of the metal in addition to reducing the size of carbideelements in the material.

In step (ii), the step of hot isostatic pressing of the component may bepreceded by a bead sintering process. In one embodiment, this processcomprises a step of putting beads (eg. about 500 micron diameter) on thesurface and then heating the component to a temperature of about 1280°C. in an inert gas (such as argon) prior to the hot isostatic pressingof the component.

In step (iii), high speed machining can be used to gradually removematerial from the component so as to bring it to the desired shape. Thecomponent can be machined with 0.1 mm step-overs between passes. Themachining preferably leaves a finely undulating surface on thecomponent. The undulations can preferably have a dimension of about 5microns from peak to trough. In one embodiment, the step of machiningthe component can be performed by a 4-axis machining centre. Thismachine runs the cutting tool over the surface following acomputer-aided design (CAD) tool path. The four axes are X, Y, Z androtation.

In step (iv) of the first aspect, the component can again undergo a hotisostatic pressing. This pressing is preferably identical to thatperformed in step (ii). This pressing preferably serves to meld theundulations so greatly reducing the requirement for removal of materialduring the subsequent polishing step (ie step (v)).

In a preferred embodiment of the process, the process can include, priorto step (v), a step of hand buffing the component. This serves to blendany roughness remaining after step (iv

In step (v), the polishing typically comprises electro-chemicalpolishing, following hand or machine physical polishing. This steppreferably removes carbides and foreign material from the surface of thecomponent as well as any residual surface scratches and asperities.

The combined steps of the process of both aspects provide an implantthat is substantially free of carbides and other impurities.Accordingly, the surface of the implant will not cause undue wear on acounter component. Furthermore, removal of carbides, removes points ofweakness for a subsequent coating and therefore provides an optimalsurface for coating. Still further, removal of carbides creates holes inthe surface of the implant, said holes acting as anchor points for anysubsequent coating. The holes may be complex in shape. Additionally, theholes may be of a pre-determined distribution and size. The distributionand size of the holes may be achieved in the second aspect by the stepof cooling the metal and particular by the step of solution annealing.

In both aspects, the component can undergo one or more of the followingfurther processing steps:

(a) oxidising at least a portion of the surface of a metal prostheticcomponent;

(b) solgel coating, or other coating means, said surface portion with analuminium oxide ceramic; and

(c) pressing the component at a relatively elevated temperature to bindthe coating to said surface portion.

These additional steps of coating the surface with a ceramic improvesthe bearing surface as it reduces the wear rate of the surface. Forexample, it preferably reduces the wear rate of the femoral componentwhen bearing against a polyethylene liner in an artificial kneeprosthesis. If desired, the bearing surface can be used to articulatewith another ceramic or ceramic-coated surface.

In step (b), the surface is preferably solgel coated with an aluminiumoxide slurry. This is seeded with alpha particles, ie tiny ceramicgrains which promote the crystallisation of the slurry. This is appliedby dipping the component in the slurry in an argon flushed vacuumchamber. The slurry serves to fill any defects in the surface of thecomponent. As discussed above, these defects are usually left by thecarbide elements removed in the polishing step (v). The depth of thesedefects and their distribution on the surface of the component can beadjusted through the solution annealing portion of the heat treatment(ii). Without solution annealing the carbides (and therefore the defectsleft when they are removed) tend to be larger and more widely dispersed.Following solution annealing they are finer and more densely arranged onthe surface. The coating is preferably between about 5 and 10 micronsbut could be made thicker or thinner if desired.

In step (c), the component preferably undergoes a hot isostaticpressing. During this pressing, the pressure can be adjusted to ensurethe metal component neither shrinks nor expands as the temperature risesso that there is only a relatively small or zero stress between theceramic layer and the metal substrate.

In step (c), the ceramic coating can be heated to a temperature suitableto fully crystallise the ceramic used in the process. In one embodiment,the temperature can be about 115° C.

According to a third aspect, the present invention is a prostheticimplant component formed using the process of:

(i) forming a prosthetic component having a shape at least approximatingthe desired final shape of the component from a metal alloy;

(ii) subjecting the component to a relatively elevated temperature andpressure to relatively reduce the grain and pore size of the metal;

(iii) machining the surface of the component;

(iv) again subjecting the machined component to a relatively elevatedtemperature and pressure; and

(v) polishing the surface of the component.

According to a fourth aspect, the present invention is a prostheticimplant component formed using the process of:

(i) forming a prosthetic component having a shape at least approximatingthe desired final shape of the component from a metal alloy;

(ii) subjecting the component to a relatively elevated temperature andpressure followed by a cooling regime;

(iii) machining the surface of the component;

(iv) polishing the surface of the component.

In one embodiment of the third and fourth aspects, the prostheticcomponent formed using the process can comprise a femoral component of aknee replacement prosthesis.

According to a fifth aspect, the present application is directed to afurther invention of forming a ceramic-coated metal prosthetic implantcomponent, the process comprising the steps of:

(a) oxidising at least a portion of the surface of a metal prostheticcomponent;

(b) solgel coating said surface portion with amceramic; and

(c) pressing, the component at a relatively elevated temperature to bindthe coating to said surface portion.

In this aspect, these steps of coating the surface with a ceramicimproves the bearing surface as it reduces the wear rate of the surface.In particular, the coating preferably serves to reduce the wear rate ofthe component when used to bear against polyethylene. If desired,however, the bearing surface can be used to articulate with, anotherceramic or ceramic-coated surface.

In step (b) of this further aspect, the surface is preferably solgelcoated with an aluminium oxide slurry. Other suitable ceramics for usein the process and means of, achieving the coating can also beenvisaged. This is seeded with alpha particles, ie tiny ceramic grainswhich promote the crystallisation of the slurry. This is applied bydipping the component in the slurry in an argon flushed vacuum chamber.The slurry serves to fill any defects in the surface of the component.The coating is preferably between about 5 and 10 microns but could bemade thicker or thinner if desired.

In step (c) of the further aspect, the component preferably undergoes ahot isostatic pressing. During this pressing, the pressure can beadjusted to ensure the metal component neither shrinks nor expands asthe temperature rises so that there is only a relatively small or zerostress between the ceramic layer and the metal substrate.

In step (c) of the further aspect, the ceramic coating can be heated toa temperature suitable to fully crystallise the ceramic used in theprocess. In one embodiment, the temperature can be about 1150° C.

According to yet a further aspect, the present invention is aceramic-coated prosthetic implant component formed using the method asdefined ion the fifth aspect.

The ceramic-coated prosthetic component can comprise a ceramic coatedfemoral component of a knee replacement prosthesis.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, a preferred embodiment of the invention is nowdescribed with reference to the accompanying drawings, in which:

FIG. 1 is a SEM of a commercially available implant showing surfacedefects;

FIG. 2 is a SEM of an implant surface after the polishing treatmentaccording to the present invention; and

FIG. 3 is a flowchart depicting the steps of the process according tothe present invention.

PREFERRED MODE OF CARRYING OUT THE INVENTION

The device formed by the method according to this invention may be usedin a range of arthroplasty procedures, but is of particularapplicability to arthroplasty procedures involving the knee joint.

A process of forming a prosthetic implant, such as femoral component ofan artificial knee joint, according to the present invention isgenerally depicted as 10 in FIG. 3.

The process 10 comprises the step of casting a CrCoMo alloy prostheticcomponent having a shape at least approximating the desired final shapeof the component. For example, the shape can approximate the shape ofthe femoral component of a knee joint. In the depicted embodiment of theprocess, the component is cast such that about 400 microns needs to bemachined from the component.

The cast component then undergoes a hot isostatic pressing and solutionannealing 20 to relatively reduce the grain and pore size of the metal.This pressing can be undertaken with a bead sintering process.

Following step 20, the process includes a step of machining the surface30 of the component. In step 30, high speed machining is used togradually remove material from the component so as to bring it to thedesired shape. In depicted step 30, the component is machined with about0.1 mm step-overs between passes. The machining leaves a finelyundulating surface on the component. The undulations resulting from thisstep have a dimension of about 5 microns from peak to trough.

Polishing step 40 comprises electrochemical polishing of the componentto remove carbides and foreign material from the surface of thecomponent. FIG. 2 depicts the vastly improved nature of the surface ofthe component using the method defined, herein.

Steps 20 to 40 constitute the process according to the second aspect ofthe invention described above. This process results in a polishedprosthetic component that is of a shape ready for packaging and use.While one example of a component according to the present invention is afemoral component of an artificial knee replacement, other suitableprosthetic components can be envisaged being formed using the method.

In the depicted embodiment, the component also undergoes additionalprocessing steps. While not depicted alone, it will be appreciated thatthe following steps constitute an invention in their own right asdefined above.

The additional processing steps comprise:

(a) oxidising at least a portion of the surface of a metal prostheticcomponent (step 50);

(b) solgel coating said surface portion with an aluminium oxide ceramic(step 60); and

(c) pressing the component at a relatively elevated temperature to bindthe coating to said surface portion (step 70).

These additional steps of coating the surface with a ceramic improvesthe bearing surface as it reduces the wear rate of the surface againstpolyethylene. If desired, the bearing surface can be used to articulatewith another ceramic or ceramic-coated surface.

In step 60, the surface is solgel coated with an aluminium oxide slurry.This is seeded with alpha particles, ie tiny ceramic grains whichpromote the crystallisation of the slurry. This is applied by dippingthe component in the slurry in an argon flushed vacuum chamber. Theslurry serves to fill any defects in the surface of the component. Theformed coating is between about 5 and 10 microns but could be madethicker or thinner if desired.

In step 70, the component preferably undergoes a hot isostatic pressing.During this pressing, the pressure can be adjusted to ensure the metalcomponent neither shrinks nor expands as the temperature rises so thatthere is only a relatively small or zero stress between the ceramiclayer and the metal substrate.

In step 70 the ceramic coating can be heated to a temperature of about1150° C.

Use of the process 10 results in the formation of a ceramic coatedprosthetic implant having the desired shape and surface properties.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1-42. (canceled)
 43. A process of forming a prosthetic implantcomponent, the process comprising the steps of: (i) forming a prostheticcomponent having a shape at least approximating the desired final shapeof the component from a metal alloy; (ii) subjecting the component to arelatively elevated temperature and pressure to relatively reduce thegrain and pore size of the metal; (iii) machining the surface of thecomponent; (iv) again subjecting the machined component to a relativelyelevated temperature and pressure; and (v) polishing the surface of thecomponent.
 44. A process of forming a prosthetic implant component, theprocess comprising the steps of: (i) forming a prosthetic componenthaving a shape at least approximating the desired final shape of thecomponent from a metal alloy; (ii) subjecting the component to arelatively elevated temperature and pressure followed by a coolingregime which comprises solution annealing; (iii) machining the surfaceof the component; (iv) polishing the surface of the component.
 45. Theprocess of claim 43 wherein at least one or more of the process stepsare performed as part of a batch process.
 46. The process of claim 43wherein the metal alloy is a chromium alloy.
 47. The process of claim 46wherein the metal alloy is a CrCoMo alloy.
 48. The process of claim 47wherein the metal alloy has the following composition: Carbon C 0.35%max. Chromium Cr 27.00-30.00% Silicon Si 1.00% max. Iron Fe 0.75% max.Nickel Ni 0.75% max. Manganese Mn 1.00% max. Molybdenum Mo 5.00-7.00%max.    Cobalt Co remainder


49. The process of claim 43 wherein in step (i), the component is formedby casting the metal alloy in the shape at least approximating thedesired final shape.
 50. The process of claim 49 wherein the componentis cast in step (i) such that a layer of no greater than about 400microns needs to be machined from the component.
 51. The process ofclaim 43 wherein in step (ii), the component undergoes a hot isostaticpressing.
 52. The process of claim 51 wherein the hot isostatic pressingis performed at about 1280 degrees Celsius in an inert gas at about 100atmospheres of pressure.
 53. The process of claim 51 wherein in step(ii), the step of hot isostatic pressing of the component is preceded bya bead sintering process.
 54. The process of claim 53 wherein the beadsintering process comprises a step of positioning beads on the surfaceof the component and then heating the component to a temperature ofabout 1280° C. in an inert gas prior to the hot isostatic pressing ofthe component.
 55. The process of claim 54 wherein the beads have adiameter of about 500 microns.
 56. The process of claim 54 wherein theinert gas is argon.
 57. The process of claim 43 wherein in step (iii),high speed machining is used to relatively gradually remove materialfrom the component so as to bring it to the desired shape.
 58. Theprocess of claim 57 wherein the component is machined with about 0.1 mmstep-overs between passes.
 59. The process of claim 58 wherein themachining leaves an undulating surface on the component, the undulationshaving a dimension of about 5 microns from peak to trough.
 60. Theprocess of claim 44 wherein the solution annealing comprises cooling thecomponent relatively rapidly, in 100 degree steps to a temperature below800 degrees Celsius.
 61. The process of claim 60 wherein the solutionannealing step substantially reduces the size of carbide elements in thecomponent.
 62. The process of claim 61 wherein the size and distributionof the carbide elements is predetermined and controlled by the solutionannealing.
 63. The process of claim 43 wherein the polishing stepsubstantially removes carbide elements from the component.
 64. Theprocess of claim 61 wherein the polishing step compriseselectro-chemical polishing of the component.
 65. The process of claim 43wherein the component is subject to an additional processing step ofoxidising at least a portion of the surface of a metal prostheticcomponent.
 66. The process of claim 65 wherein the component is subjectto an additional processing step of solgel coating said surface portionwith an aluminium oxide ceramic.
 67. The process of claim 66 wherein thecomponent is subject to an additional processing step of pressing thecomponent at a relatively elevated temperature to bind the coating tosaid surface portion.
 68. The process of claim 67 wherein said surfaceportion is the bearing surface of a component that is to be used toarticulate with another ceramic or ceramic-coated surface.
 69. Theprocess of claim 66 wherein the surface is solgel coated with analuminium oxide slurry that is seeded with ceramic grains which promotethe crystallisation of the slurry.
 70. The process of claim 69 whereinthe slurry is applied to the surface of the component by dipping thecomponent in the slurry in an argon flushed vacuum chamber.
 71. Theprocess of claim 67 wherein the additional processing step comprises thecomponent undergoing a hot isostatic pressing.
 72. The process of claim71 wherein the relatively elevated temperature is about 1150° C.
 73. Aprosthetic implant component formed using the process of: (i) forming aprosthetic component having a shape at least approximating the desiredfinal shape of the component from a metal alloy; (ii) subjecting thecomponent to a relatively elevated temperature and pressure torelatively reduce the grain and pore size of the metal; (iii) machiningthe surface of the component; (iv) again subjecting the machinedcomponent to a relatively elevated temperature and pressure; and (v)polishing the surface of the component.
 74. A prosthetic implantcomponent formed using the process of: (i) forming a prostheticcomponent having a shape at least approximating the desired final shapeof the component from a metal alloy; (ii) subjecting the component to arelatively elevated temperature and pressure followed by a coolingregime which comprises solution annealing; (iii) machining the surfaceof the component; (iv) polishing the surface of the component.
 75. Theprosthetic implant of claim 73 wherein the prosthetic component is afemoral component of a knee replacement prosthesis.
 76. A process forforming a ceramic-coated metal prosthetic implant component, the processcomprising the steps of: (a) oxidising at least a portion of the surfaceof a metal prosthetic component; (b) solgel coating said surface portionwith a ceramic; and (c) pressing the component at a relatively elevatedtemperature to bind the coating to said surface portion.
 77. The processof claim 76 wherein said surface portion is the bearing surface of acomponent that is to be used to articulate with another ceramic orceramic-coated surface.
 78. The process of claim 76 wherein in step (b),the surface portion is solgel coated with an aluminium oxide slurry thatis seeded with ceramic grains which promote the crystallisation of theslurry.
 79. The process of claim 78 wherein the slurry is applied to thesurface of the component by dipping the component in the slurry in anargon flushed vacuum chamber.
 80. The process of claim 76 wherein instep (c), the additional processing step comprises the componentundergoing a hot isostatic pressing.
 81. The process of claim 80 whereinthe relatively elevated temperature is about 1150° C.
 82. Aceramic-coated prosthetic implant component when formed using the methodas defined in claim
 76. 83. The ceramic-coated prosthetic implantcomponent of claim 82 wherein the component comprises a ceramic-coatedfemoral component of a knee replacement prosthesis.